L0301P77 - Necrosis and Apoptosis
Cell Injury and Death Injury *can be reversible as long as: **reversible morphological changes **membrane is not severely damaged **compartmentalisation of organelles is not compromised Death *is associated with irreversible injury **necrosis ***always pathological ***changes occur through digestion or dissolution of dead cellular elements **apoptosis ***can be pathological or physiological ***eliminates “unwanted cells” ***initiation of “programmed cell death” Causes of Injury or Death *hypoxia/ischaemia *physical/chemical trauma *microbial infections *immune/autoimmune reactions *metabolic/nutritional imbalances and deficiencies *ageing Potential Mechanisms of Cell Death *depletion of ATP *mitochondrial damage *loss of calcium homeostasis **normal cytoplasm concentration = 0 *production of reactive oxygen species (ROS) - oxygen derived free radicals Necrosis and Apoptosis Summary of Differences Necrosis *death of contiguous cells in an organ - not just a single cell Morphology *cellular swelling or rupture *denaturation of cytoplasmic proteins *breakdown of cell organelles *breakdown of nucleus and nuclear DNA Process #pyknosis - nuclear shrinkage; DNA condense #karyorrhexis - fragmentation: nuclear membrane ruptures #karyolysis - fading: dissolution of chromatin via DNAases and RNAases Patterns of Necrosis *coagulative necrosis **proteins denature and aggregate rather than degrade *liquefactive necrosis **enzymic digestion of cellular components *caseous necrosis *fat necrosis *fibrinoid necrosis **fibrin accumulation *gangrenous necrosis Coagulative Necrosis *tissue shape/architecture is maintained for a period of time *damages proteins and digestive enzymes **thus dead cells cannot be digested *macroscopic: regions of pallor, firm texture *inflammation and phagocytosis initiated *eventually, dead cells and tissue will be replaced by regeneration (if possible) or fibrous scar tissue *e.g. infarcts in solid tissues (heart, kidney) Liquefactive Necrosis *complete dissolution of tissue *not all proteins have been destroyed *site of necrosis marked by a cavity; regeneration cannot occur *dead cells completely digested and phagocytosed *inflammatory response initiated *e.g. cerebral infarction **liquefactive in brain because of lack of supportive stroma Caseous Necrosis *typically caused by tuberculosis infection *tissue architecture is abolished and viable cells are no longer recognisable *accumulation of amorphous (no structure) cheese‐like debris within an area of necrosis *inflammation and phagocytosis initiated Fat Necrosis *occurs after the release of intracellular fat *most common: trauma causing release of activated pancreatic lipases into the pancreas and peritoneal cavity *lipases cause fat cells to die by dissolving all the fat within the tissue *free fatty acids accumulate and precipitate as calcium soaps (saponification) *inflammatory response initiated *e.g. acute pancreatitis Fibrinoid Necrosis *occurs in blood vessels following a strong immune response *complexes of antibodies and antigen are deposited in wall of arteries/blood vessels *this deposit, along with fibrin, result in a amorphous “fibrinoid” appearance *inflammatory response initiated Gangrenous Necrosis *loss of blood/oxygen/energy supply to limb **usually a lower limb *dry gangrene - undergoes coagulative necrosis across multiple layers of tissue *wet gangrene – liquefactive necrosis *inflammatory response initiated   Mechanisms for Necrosis Depletion of ATP / Mitochondrial Damage *major causes of ATP depletion: **hypoxia and ischaemia *ATP is required for all processes in cells **synthesis /degradation / membrane transport / ion gradients Mechanisms *failure of the Na+/K+ ATPase **results in increase of intracellular Na+ **gain of solute is followed by gain in water - ***cell swelling **compensatory increase in anaerobic glycolysis —> decreased cellular pH —> impairment of enzymic function *failure of ATP‐dependent Ca2+ pump **results in influx of Ca2+ *structural damage to protein synthesis apparatus **e.g. detachment of ribosomes —> failure in protein synthesis Loss of Calcium Homeostasis *intracellular calcium concentration is normally 0, an influx of CA2+ will result in activation of Ca2+ dependent enzymes Ca2+ Dependent Enzymes *activation of proteases **e.g. calpain **breakdown cytoskeletal proteins *activation of lipases **e.g. phospholipases ***breakdown of membrane phospholipids ***activation of arachidonic acid cascades *activation of endonucleases *activation of production of oxygen derived free radicals *activation of (neuro)inflammatory processes Reactive Oxygen Species (ROS) and Oxygen Derived Free Radicals *occurs often in ischaemia-reperfusion injuries (return of oxygen after ischaemia) Oxygen Free Radicals *superoxide (O2) **also generated in neutrophils and macrophages *hydroxy free radicals (OH‐) *ONOO‐ (peroxynitrite) derived from NO (nitric oxide) and O2‐) Mechanism *ROS attack key molecules **enzymes, membrane lipids, DNA and mitochondria *hydroxy free radicals and peroxynitrite are most destructive Defence Mechanisms *defence against ROS include antioxidant enzymes and enzyme systems: **SOD (superoxide dismutase) **catalase **glutathione peroxidase Apoptosis *programmed cell death - normal physiology Occurs *especially during foetal development *in response to hormonal cycles **e.g. endometrium *normal turn over in proliferating tissues (e.g.intestinal epithelium) **cells shrink, not swell **nuclei condense and DNA fragments **cells fragment into membrane‐bound bits **phagocytosed by macrophages Causes Physiological *eliminate cells that are no longer needed and to maintain a steady number of various cells populations in tissues Pathological *eliminates cells that are injured beyond repair without eliciting a host reaction, thus limiting collateral tissue damage *damage may include **DNA damage **cytotoxic drugs **free radical bursts **hypoxia Apoptotic Pathways *apoptosis results from the activation of caspases and leads to two pathways Intrinsic Pathway *also known as the mitochondrial pathway *activated by DNA damage (and withdrawal of cell survival factors) *DNA damage activates p53 pathway and the activation of pro‐apoptotic factors *pro‐apoptotic factors **promote release of cytochrome c from the mitochondria which activates caspase‐9 leading to cell death **such as Bax *anti-apoptotic factors **in normal cells, survival factors activate anti‐apoptotic factors **thus withdrawal of survival factors activates apoptosis **includes Bcl‐2 Extrinsic Pathway *also known as the death receptor pathway *TNF receptors **found on all cells **responds to and is activated by FasL ***protein expressed on membranes of cytotoxic T‐lymphocytes initiating killing of target cells Caspase-3 *is the final mediator of apoptosis and is at the effector stage of apoptosis *when activated it cleaves and inactivates many cell constituents, inluding: **DNA repair enzymes **structural constituents *results in the cell being reduced to a cluster of membrane bound bodies that are phagocytosed by macrophages